1. Field of the Invention
The present invention relates to a laser wavelength conversion device incorporated with a laser wavelength conversion element, in which a polarization structure of a non-linear optical single crystal substrate is periodically reversed, and a method for forming a polarization reversed structure. The present invention also relates to an image display device incorporated with the laser wavelength conversion device.
2. Description of the Background Art
Applying an electric field to a single-polarized ferroelectric crystal in a direction opposite to a polarization direction enables to reverse the polarization direction of a portion where the electric field is applied. As recited in Japanese Unexamined Patent Publication No. 2004-246332, applying an electric field with use of a periodical electrode enables to alternately form a region where the polarization is reversed and a region where the polarization is not reversed, thereby enabling to form a periodically polarization reversed structure.
A ferroelectric crystal having the aforementioned periodically polarization reversed structure is operable to convert a fundamental wave laser beam, which is incident onto the ferroelectric crystal in a periodically reversed polarization direction, into a laser beam having a wavelength different from the wavelength of the incident fundamental wave. The ferroelectric crystal having the aforementioned periodically polarization reversed structure is widely used as a quasi phase matching (QPM) wavelength conversion element for converting the wavelength of a laser beam.
Also, in recent years, there is a demand for developing a high-output laser wavelength conversion technique capable of high-output laser output of several watts or more for laser display or laser processing.
It is highly likely that a laser beam of an unduly large light intensity may cause crystal damage or degradation in a wavelength conversion element. It is possible to suppress crystal damage or degradation by increasing the beam diameter of a laser beam to be used in wavelength conversion and suppressing an increase in light intensity. There is, however, a limit in increasing the width of polarization direction of a crystal capable of uniformly forming a periodically polarization reversed structure. Thus, increasing the width of polarization direction of a crystal has become a challenging task to perform.
FIG. 15 is a diagram for describing a process of forming a periodically polarization reversed structure in a conventional laser wavelength conversion element. FIG. 16 is a diagram showing the conventional laser wavelength conversion element having the periodically polarization reversed structure. FIG. 17 is a diagram showing relations between a position of a polarization direction and a wavelength conversion efficiency in the conventional laser wavelength conversion element.
As shown in FIG. 15, a polarization reversed region starts from a +Z surface 312 of a ferroelectric crystal 311, and spreads in lateral directions (X-directions) and in −Z direction. In the case where a periodically polarization reversed structure is formed by periodical electrodes 313 formed on the +Z surface 312, and an opposing electrode 315 formed on a −Z surface 314, wedge-shaped polarization reversed portions 316 are started to be formed at a position beneath the periodical electrodes 313. As an electric field is applied to the ferroelectric crystal 311, the polarization reversed portions 316 grow, and as shown in FIG. 16, distal ends 317 of the polarization reversed portions 316 reach close to the −Z surface 314.
As shown in FIG. 16, a laser beam 319 as a fundamental wave exiting from a light source 318 is incident onto the ferroelectric crystal 311 having the periodically polarization reversed structure in a periodically reversed polarization direction to convert a part of the laser beam 319 into a laser beam 320 having a wavelength different from the wavelength of the incident fundamental wave. When the above wavelength conversion is performed, a wavelength conversion efficiency a depends on a ratio of the polarization reversed portions 316 and a polarization non-reversed portion 321 on a beam path of the fundamental wave. The wavelength conversion efficiency σ is maximum when the ratio is equal to 1:1. Specifically, assuming that the ratio of the polarization reversed portions 316 relative to the entirety of the wavelength conversion element is D (duty ratio [%]), the wavelength conversion efficiency σ is expressed by the following formula (1):σ∝ sin2(D/100)  (1)
Further, the polarization reversed structure has a series of wedge shapes, as shown in FIG. 16, in other words, is expressed by a graph showing a monotonous increase or a monotonous decrease, with a Z-coordinate axis in horizontal axis and a duty ratio in vertical axis. The duty ratio is decreased from 100% to 0% in a direction from the +Z surface 312 of the crystal 311 to the −Z surface 314 thereof. Assuming that the thickness of the crystal 311 is 1 mm, as shown in FIG. 17, the wavelength conversion efficiency is maximum at a middle portion of the crystal 311 in Z-axis direction, and is reduced to about a half of the maximum at a position away from the middle portion in Z-axis direction by about ±250 μm.
The laser wavelength conversion element described in the above example is a laser wavelength conversion element having a period length of 7 μm. The ratio of the growing speed of the polarization reversed portion in Z-axis direction, and the growing speed of the polarization reversed portion in lateral direction is substantially the same among the polarization reversed portions. Accordingly, a half bandwidth of wavelength conversion efficiency of a laser wavelength conversion element having a shorter period is decreased in proportional to the period.
Similarly to the above, even in the case where a crystal with a thickness exceeding 1 mm is used, wedge shapes of a polarization reversed structure, in other words, the gradient of a graph having a Z-coordinate axis in horizontal axis and a duty ratio in vertical axis is not changed.
For the aforementioned reasons, even if a fundamental wave laser beam having a large beam diameter of several hundred micrometers or more is subjected to wavelength conversion, the region of the crystal subjected to high-efficiency wavelength conversion is limited. As a result, the beam diameter of a laser beam to be generated by wavelength conversion in Z-axis direction is limited.
In the case where wavelength conversion is performed by using a high-output laser, it is highly likely that the light intensity of an incident laser beam or a laser beam generated by wavelength conversion may be unduly increased, which may cause crystal damage or degradation. It is possible to suppress crystal damage or degradation by increasing the beam diameter of a laser beam to be used in wavelength conversion, and suppressing an increase in light intensity. There is, however, a limit in increasing the width of a crystal in Z-axis direction, while securely and uniformly forming a periodically polarization reversed structure. Accordingly, it is impossible to increase the beam diameter over the limit.
In the case where a green laser beam is obtained by subjecting an infrared laser beam to wavelength conversion, using a laser wavelength conversion element, an ultraviolet laser beam is generated by superimposing the green laser beam and the infrared laser beam. When the ultraviolet laser beam and the green laser beam are superimposed, the green laser beam is absorbed, and crystal damage may occur. In view of the above phenomenon, it is indispensable to reduce the infrared beam intensity by increasing the beam diameter in order to realize high-output performance, because generally, the intensity of green beam is proportional to the square of the incident beam intensity, and the intensity of ultraviolet beam is proportional to the third power of the incident beam intensity.